The present invention relates to an electron multiplier for a photomultiplier tube and more particularly to an electron multiplier having an improved planar ultimate dynode and a planar anode structure.
One embodiment of a conventional planar anode structure is shown in U.S. Pat. No. 2,200,722 issued to Pierce et al. on May 14, 1940. In the Pierce et al. structure, the anode comprises a metallic plate of relatively small area disposed within a screen electrode adjacent to the ultimate dynode. The anode is supported by a pair of bent wires extending through and fitted in suitable apertures in a pair of insulating uprights. The screen electrode operates at penultimate dynode potential to focus the electrons onto the anode. The flat anode plate supported on a pair of wires is neither mechanically stable nor inflexible and thus the anode may flex under temperature variations resulting in output signal instability or time-spread. Furthermore, the high field gradient arising from operating the anode within a screen electrode, which is at penultimate dynode potential, creates the possibility of high electrical leakage current to the anode.
An equally unsatisfactory anode configuration is shown in U.S. Pat. No. 2,245,624 issued to Teal on June 17, 1941. In one embodiment of the Teal patent, the anode comprises a metallic base plate supported from a cross-wire by a pair of rigid rods or wires. A second metallic plate is affixed to the base plate by welding. The second plate extends at an acute angle toward the ultimate dynode to form a collector. The Teal structure is unstable and difficult to manufacture in a reproducible manner.
U.S. Pat. No. 2,866,914 issued to Lallemand on Dec. 30, 1958 shows a wire grid anode disposed within a "Faraday Box". The anode is supported at one side by a lead-in wire that extends through an aperture in the side of the "Faraday Box". It is alleged that the anode is supported firmly; however, the single point of support located at one edge of the grid permits vibration or flexing of the anode under all but ideal operating conditions.
Thus, there is a need for a highly stable, simple planar anode and planar ultimate dynode structure having a minimum of electrical leakage between the two elements.